1. Field of the Invention
This invention relates to semi permeable membranes for the separation of fluid mixtures. More particularly, it relates to the enhancing of the separation characteristics of composite membranes.
2. Description of the Prior Art
Semi-permeable membranes capable of selectively permeating one component of a fluid mixture, either liquid or gas, are considered in the art as a convenient, potentially highly advantageous means for achieving desirable fluid separations. Such membranes, for practical commercial application, must be capable of achieving an acceptable selectivity of separation of the gases or liquids contained in a feed gas mixture while, at the same time, achieving a desirably high flux, or permeability, of the fluid component being selectively permeated through the membranes.
Semi-permeable membranes have been extensively used in reverse osmosis or ultrafiltration processes, as in the desalination of water. In the reverse osmosis process, high pressure saline water is placed in contact with a semi-permeable membrane that is permeable to water, but relatively impermeable to salt. Concentrated brine and relatively pure water are separated thereby.
Semi-permeable membranes are also utilized in separation processes that involve a phase change of one or more components of the mixture to be separated. Thus, the feed and the permeate streams are alternately in the liquid and gaseous state, with gas being present on one side of the membrane. An example of such a process is pervaporation through membranes, which is particularly useful in the separation of liquids from their azeotrope solvent mixtures, and wherein liquid is present on the feed side of the membrane. Another such process is perstruction, wherein liquid is present on the permeate side of the membrane.
More recently, semi-permeable membranes have found broad utilization with respect to a variety of gas separation applications. Examples of such applications include air separation, the recovery of hydrogen from ammonia purge gas and from refinery gas streams, and carbon dioxide and methane separations in various operations such as tertiary oil recovery.
Semi-permeable membranes of a variety of materials and forms have been proposed in the art for the carrying out of such separations. So called composite membranes have been proposed wherein a thin layer of a suitable semi-permeable membrane material is superimposed on a relatively porous substrate. The separation layer is advantageously very thin in order to provide the desirably high flux referred to above. The substrate provides support for the delicate, very thin permeable membrane layer or coating superimposed thereon. Such composite membrane structures are described in the Klass et al. U.S. Pat. No. 3,616,607.
It will be appreciated that membranes for such separation processes, both liquid and gas, require membranes possessing a high degree of perfection in the membrane, or separation, layer. In gas separation processes, pervaporation, perstruction and the like, the best results would obviously be obtained if the membrane layer were free of any residual pores or other imperfections. On the other hand, the membrane layer needs to be made as thin as possible in order to attain desirably high permeation rates, and thus high overall separation process productivity. In such circumstances, the presence of morphological imperfections are frequently found to occur in the thin separation layer of membrane assemblies in the form of residual pores, minute pinholes and the like. Such imperfections can be introduced into the membrane system in the course of the various typical membrane manufacturing steps, such as spinning, casting, coating, curing and membrane module manufacturing operations.
In the field of reverse osmosis desalination processes, it is customary to treat membranes that exhibit subperformance separation characteristics, or membrane modules that have deteriorated during use in desalination operations in the field, with supplementary treating solutions. Such treatments are typically carried out from water solutions and result in the deposition of small amounts of materials dissolved in such solutions onto the exterior surface, and into the exterior pores, of the reverse osmosis membrane. Such treatments, described in the Ganci et al. U.S. Pat. No. 3,808,303, and in various other references, apparently result in modification of the surface characteristics and pore sizes of reverse osmosis membranes, with subsequent increase in membrane separation performance.
To overcome the problem of defects in the field of gas separation membrane manufacturing, the Browall U.S. Pat. No. 3,980,456, has disclosed the application of a second, sealing coating over the very thin membrane to cover defects caused by particulate impurities. Such treated composite structures are complex in nature and, moreover, the use of a superimposed very thin membrane on a porous support substrate has not generally provided the desired selectivity of separation without an unacceptable reduction in the flux, or permeability, of the permeate gas.
The problem of membrane defects has been experienced not only with respect to composite membranes, but also with respect to asymmetric type membranes. Such asymmetric membranes are distinguished by the existence of two distinct morphological regions within the membrane structure. One such region comprises a thin, dense, semi-permeable skin capable of selectively permeating one component of a gas mixture. The other region comprises a less dense, porous, non-selective support region that serves to preclude the collapse of the thin skin region of the membrane under pressure. Such asymmetric membranes are described in the Loeb et al. U.S. Pat. No. 3,133,132. As in the case of the composite membranes described above, such asymmetric membranes, when applied in gas separation operations, are frequently not sufficiently perfect for such purposes, and contain imperfections. A significantly reduced amount of gas separation will occur as a result of the presence of such defects in the asymmetric membrane structure. The Henis et al. U.S. Pat. No. 4,230,463, discloses a proposed solution to the problem caused by such defects. In the approach of Henis et al., the asymmetric membrane having limited amounts of residual surface porosity, usually less than 10.sup.-6 of the total surface area, is coated to cure defects therein. The coating material employed has a determined intrinsic separation factor that is less than that of the material of the asymmetric membrane. The resulting multi component membrane exhibits a separation factor significantly greater than the determined intrinsic separation factor of the material of the coating and greater than the separation factor exhibited by the uncoated asymmetric membrane.
In the Ward U.S. Pat. No. 4,214,020, a process is disclosed that teaches coating the exterior surface of a hollow fiber membrane assembly by immersing a bundle of hollow fibers into a coating solution, and driving the coating solution into the fiber by applying pressure from the exterior to the interior of the hollow fibers. This process, leading to the formation of a continuous layer/coating on the exterior of the hollow fibers, is particularly useful in the preparation of highly selective gas separation membranes by the coating of asymmetric membranes having some residual porosity with coating solutions of materials highly permeable to gases, as described in the Henis et al. patent referred to above.
Another treatment procedure to improve the separation characteristics of asymmetric membranes having residual porosity is disclosed in the Lee U.S. Pat. No. 4,527,999. This procedure relates to the treating of asymmetric cellulose acetate membranes under conditions that lead to differential collapse of surface pores, resulting in greatly improved membrane separation characteristics.
While improvements have thus been made with respect to the repairing of membrane defects, the presence of such defects remains a problem, and this problem is aggravated by the increasing requirements for extremely thin external membrane separation layers in order to achieve high permeate flux. The providing of very thin membrane separation layers can frequently, in turn, lead to a decrease in the separation factor of the membrane due to the increased presence of imperfections therein, and to the collapse of the separation layer under high operating pressure conditions. A need remains, therefore, for further improvements in the art with respect to eliminating or minimizing the problem of membrane defects. In particular, it is desired that the repairing of defects be carried out so as to enable higher selectivity characteristics to be achieved without adverse affect on the permeability characteristics of the membrane, so that advantageous combinations of selectivity and permeability can be achieved in practical fluid separation operations.
The need for further improvements in the repairing of defects is particularly pertinent with respect to composite membranes. Conventional composite membranes, in which the coating provides the separation characteristics of the composite membrane structure, have an inherent flexibility of application not possessed by other forms of membrane structure. The separation layer can thus be selected particularly for a desired separation application, while the hollow fiber or other desired form of substrate provides a relatively porous, non selective support for the separation layer. As the advantages of composite membranes become increasingly appreciated with regard to a variety of gas and liquid separation operations, the desire in the art for the development of various composites for particular applications will like increase. Such development, as indicated above, involves desirable separation characteristics combined with high flux, or advantageous combinations of separation and permeability consistent with the overall performance requirements of particular applications. The providing of a convenient, effective and generally applicable means for repairing minute defects in the separation layer of such composites becomes ever more important as the applications for composites increase and as the separation layers become thinner to more fully realize the overall separation-permeability requirements pertaining to the use of composite membranes in a variety of practical, commercial gas or liquid separation applications.
It is an object of the invention, therefore, to provide a convenient, generally applicable process for the plugging of the minute defects in the thin separation layers of composite membranes.
It is another object of the invention to provide improved composite membranes capable of approaching the intrinsic separation characteristics of the coating material without any significant loss in the permeability characteristics of the separation layer.
It is a further object of the invention to provide composite membranes having an advantageous combination of selectivity and permeability for use in particular gas, pervaporation or perstruction separation operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.